1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor capacitor and a semiconductor capacitor.
2. Description of the Related Art
In a capacitor used for a DRAM, a rough-surface polysilicon film is used as a lower electrode to thereby allow an increase in capacitor surface area and an improvement in electric capacitance. Here, rough-surface polysilicon is a polysilicon film having uneven spots or projections and depressions defined in the surface thereof. Such a structure is capable of obtaining about 2 to 2.5-times electric capacitance as compared with a structure in which a polysilicon film whose surface is flat, is used as a lower electrode.
Meanwhile, the rough-surface polysilicon film needs to have conductivity with a view toward using the rough-surface polysilicon film as the lower electrode. For example, two methods to be described below have heretofore been proposed as a method of forming the rough-surface polysilicon film having the conductivity. Incidentally, the two methods to be described below both utilizes a phenomenon in which when an amorphous film composed of silicon is crystallized, crystal grains are grown on the surface thereof by nucleation and surface migration of silicon atoms.
First of all, the first method is a method of forming a non-doped rough-surface polysilicon film and introducing an impurity into the rough-surface polysilicon film. In the first method referred to above, uneven spots or projections or depressions formed in the surface of the rough-surface polysilicon film deform due to impurity introduction such as ion injection, thermal diffusion or the like. Thus, this could lead to a reduction in the surface area of the rough-surface polysilicon film. Further, the first method will incur the risk of reducing throughput at the production of a device because the impurity introducing step is indispensable.
The second method is a method of forming an amorphous film composed of impurity-doped silicon and heat-treating the amorphous film to thereby obtain a rough-surface polysilicon film. Described more specifically, in the second method, an amorphous film composed of silicon containing an impurity necessary for conductivity is first formed by reduced pressure CVD. Afterwards, a native oxide film on the surface of the amorphous film is removed and the amorphous film is heat-treated in a pressure-reduced atmosphere. The second method described above differs from the first method in that since the introduction of the impurity into the rough-surface polysilicon film is unnecessary, projections and depressions defined in the surface of the rough-surface polysilicon film are hard to deform in form.
In the conventional second method, however, the impurity already contained in the amorphous film upon heat treatment produces a bottleneck in the migration of silicon atoms. Therefore, the forms of the projections and depressions defined in the surface of the rough-surface polysilicon film will change due to the concentration of the impurity contained in the amorphous film. Thus, if the conventional second method is applied to the formation of the rough-surface polysilicon film, then the electric capacitance of the semiconductor capacitor is not stabilized. Further, since the second method needs to remove the native oxide film and additionally effect heat treatment on the amorphous film in the pressure-reduced atmosphere, it is apt to reduce throughput at device production.